An unusual point in the calculation of the wrapped D-brane contribution
is the existence of tachyonic modes, which have negative energies.
Such modes correspond to terms
in \(i\)  with negative power of \(q\) .
The existence of
such modes are related to the fact that the cycle
is topologically trivial.
Usually we take \(|q|<1\)  and other fugacities to be phases,
and if there exists such tachyonic modes
the geometric series in
(REF ) does not converge.
In [1]} it was proposed that
when the geometric series
(REF )
diverges we should define the plethystic exponential by
analytically continued form:
\(\mathop {\rm Pexp}\nolimits (f_r)=\frac{1}{1-f_r}.\) 
